Loud Engine Noise problems of the 2017 Cadillac XT5

While driving 30mph on city street engine made loud roaring noise and dramatically slowed itself down to almost a stop as if I slammed on brakes. I had vehicle(1715 mileage) towed to dealership. They kept it 2 days and told me nothing was wrong. I picked up car and while driving home the engine made loud noise and dramatically slowed down once again. The next day I took my car to another dealership hoping they could repair vehicle. They kept car for 14 days and told me they could not duplicate concern and did not repair. So I picked up car hoping it would not happen again. It did happen 3 more times. One incident happened while driving through intersection on city street engine made loud roaring noise and abruptly slowed itself down to almost a stop. But in this instance when I stepped on accelerator to drive forward to get out of intersection the car would not move,it froze . Luckily I did not get hit in the back. I waited, turned car off, and then turned it on. I was able to drive home. I took car back to dealership on 7/19/17 explaining the car was unsafe since these problems are erratic and I could get hit in the back when the vehicle slows down to a stop in the middle of traffic. They kept car for 9 days called and told me they could not find problem so they wanted their loaner back. I was afraid to drive car back home so the dealer delivered car to my house and took back loaner on 7/28/17. It has been in my garage since then since I fear for my safety and my family and will not drive car fearing I'll get hit while driving if the vehicle malfunctions again. I never get a warning of any sort before it happens. After it happens and I turn the car off and on again the vehicle seems perfect. But I never know when it will break again since it is erratic. It is definitely a safety hazard.